The present invention generally relates to the art of digital imaging. It finds particular application in conjunction with correcting illumination variation in a scanned image of a bound book, and will be described with particular reference thereto. It is to be appreciated, however, that the invention will also find application in conjunction with correction of illumination variation in other bound material and three-dimensional objects on a variety of image acquisition and output devices, such as scanners, xerographic copiers, and printers.
Conventionally, when a book or other bound, multi-page original is scanned on a scanner or xerographic apparatus, the resulting digital image is very dark in the region adjacent the binding. Such an effect is illustrated in FIG. 1. This effect occurs due to the distance and shape of the gap, as well as properties of the illuminator and imaging optics. The illumination problem varies spatially, ranging from nominal and within specification at some distance from the gap or binding, to progressively darker and unacceptable, i.e. outside of specification, for pixels closer to the center of the gap. The spatially varying illumination degrades the appearance of the scanned image, often making it unreadable.
The above illumination variation problem has been addressed in a variety of ways in the prior art. A common xe2x80x9cbrute forcexe2x80x9d approach is to press the page or pages to be copied against the surface of the platen. However, portions of the pages adjacent the binding region still remain a short distance above the platen surface.
Another solution to the above problem is to modify the platen to have a sloping edge portion whereby the bound part of the book is placed in a corner position such that the entire surface of the page being copied is in close contact with the platen surface. This system suffers from a limited magnification range because of restriction on scan component movement near the sloping corner edge. In addition, operability and production are limited by the inability to perform a xe2x80x9csplit scan,xe2x80x9d where both pages of a book are scanned without repositioning.
Another solution to the illumination variation problem involves a correction in which the height variations of the document are detected and used in a correction algorithm. This type of height-dependent correction may be employed by face-up image reading devices, where a document is scanned in a face up position, such as that embodied in U.S. Pat. No. 5,659,404 to Matsuda. Incorporating height variation data into the illumination correction algorithm adds unwanted processing complexity and time and requires additional component costs.
The present invention contemplates a new and improved adaptive illumination correction method which overcomes the above-referenced problems and others.
In accordance with one aspect of the present invention, a method of correcting illumination variation in a scanned image of a non-planar original object includes scanning at least a portion of the non-planar original object in order to produce scanned image data. Illumination variation data is extracted from the scanned image data and a plurality of illumination compensation values are derived from the acquired illumination variation data. The scanned image data is then altered in accordance with the illumination compensation values.
In accordance with a more limited aspect of the present invention, acquiring illumination variation data includes measuring illumination values at a plurality of pixel locations within a sampling window.
In accordance with a more limited aspect of the present invention, deriving a plurality of illumination compensation values includes defining a background illumination distribution and a foreground illumination distribution.
In accordance with a more limited aspect of the present invention, deriving a plurality of illumination compensation values includes determining a background low threshold value BL and a background high threshold value BH. In addition, a foreground low threshold value FL and a foreground high threshold value FH are determined.
In accordance with a more limited aspect of the present invention, deriving a plurality of illumination compensation values includes calculating a spatially dependent illumination gain factor g(x) from the measured illumination values.
In accordance with a more limited aspect of the present invention, calculating the spatially dependent illumination gain factor g(x) includes averaging no more than a rank-ordered upper twenty percent of pixel illumination values within each sampling window.
In accordance with another aspect of the present invention, a digital imaging method for imaging an open book having a book binding includes scanning the open book to produce scanned image data, where the scanned image data contains illumination variations adjacent the book binding. Illumination data is extracted from the scanned image and the illumination variations across the scanned image are normalized. The normalized image is then outputted on a physical media.
In accordance with a more limited aspect of the present invention, the normalizing step includes defining a background and a foreground illumination distribution from the obtained light levels. From each background illumination distribution, a background low threshold value BL and a background high threshold value BH are determined. From each foreground illumination distribution, a foreground low threshold value FL and a foreground high threshold value FH are determined.
In accordance with a more limited aspect of the present invention, the normalizing step further includes calculating reference values for BL, BH, FL, and FH and generating a tone reproduction curve (TRC) corresponding to the calculated reference values of BL, BH, FL, and FH.
In accordance with another aspect of the present invention, a xerographic apparatus for reproducing an image representation of a scanned open book includes a planar imaging platen and means for scanning the open book placed on the platen to produce scanned image data, where the scanned image data contains illumination variations adjacent the book binding. The apparatus further includes means for extracting illumination data from the scanned image data and a processor for calculating a plurality of illumination compensation parameters from the extracted illumination data. An image processor compensates the scanned image data for illumination variations and an illuminator transfers an image charge pattern to a photoreceptor. At least one developing station develops the charge pattern and at least one fuser station fixes a developed image onto a physical media.
In accordance with a more limited aspect of the present invention, the processor for calculating a plurality of illumination compensation parameters includes means for defining a background illumination distribution and a foreground illumination distribution and means for determining high and low threshold values for the background and foreground illumination distributions.
In accordance with a more limited aspect of the present invention, the image processor includes means for generating a tone reproduction curve (TRC) corresponding to the determined high and low threshold values for the background and foreground illumination distributions in sampling positions not adjacent the book binding. The image processor further includes means for mapping the scanned image data in accordance with the generated TRC.
One advantage of the present invention resides in an illumination correction without external height sensing.
Another advantage of the present invention resides in an illumination correction without use of calibration feature detection.
Another advantage of the present invention resides in improved scanning of a bound object without illumination variation adjacent the binding.
Still other benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments.